A friend of mine recently called with a question about the cover depth requirement for an underground 120-volt (V) lighting circuit installation in rigid polyvinyl chloride (PVC) conduit supplying a wet-niche luminaire in a swimming pool. Initially, the answer seemed relatively straightforward with a quick reference to 680.10 of the 2011 National Electrical Code (NEC). However, new text has been added to Table 680.10 in the 2011 NEC dealing with the minimum cover depths for raceways installed within 5 feet (1.5 meters) of the edge of a swimming pool. Research of 680.10 and the accompanying table was required to thoroughly understand the burial depth and the intent of the existing text as well as the history of the requirement and the new text provided in the 2011 NEC.
Section 680.10 was first provided in the 1981 NEC as text format without a table and consisted of a few short lines addressing underground wiring location with two exceptions. The text was as follows: “Underground wiring shall not be permitted under the pool or under the area extending 5 ft (1.5 m) horizontally from the inside wall of the pool.” The purpose of the original text was to keep all wiring away from the pool so induction, capacitance, leakage current and shorted conductors could not adversely affect people swimming in the pool or walking on the deck surrounding the pool, and that was stated in the substantiation. Two exceptions accompanied the mandatory text. The first exception stated, “wiring necessary to supply pool equipment permitted by this article (680) shall be allowed within this area.” There was anecdotal information about a woman who received a shock while swimming, and directly buried cables located in the ground in proximity to the gunite concrete pool were blamed.
The second exception inserted in the 1981 NEC stated, “When space limitations prevent wiring from being routed 5 feet (1.52 m) or more from the pool, such wiring shall be permitted when installed in rigid metal conduit, intermediate metal conduit, or a nonmetallic raceway system. All metallic conduits shall be corrosion-resistant and suitable for the location. The exceptions in Section 300-5(a) shall not apply.” The raceways provided protection for the conductors and reduced the possible hazard to swimmers. This second exception to 680.10 did not apply to raceways supplying pool electrical equipment (that was covered by the first exception), only electrical equipment not related to the pool. There were no depth requirements in the 1981 NEC, except those in Table 300-5, which required rigid nonmetallic conduit (approved for direct burial without concrete encasement) to be buried 18 inches deep. However, an exception in 300-5 permitted the depth of a raceway to be reduced by 6 inches for every 2 inches of concrete placed over the underground installation. This exception in 300-5 did not apply to the raceways containing conductors for equipment not associated with the pool, as noted in 680-10, Exception No. 2.
Fast forward to the 2008 NEC where there is text and a table as follows: “Underground wiring shall not be permitted under the pool or within the area extending 1.5 m (5 ft) horizontally from the inside wall of the pool unless this wiring is necessary to supply pool equipment permitted by this article.” Former Exception No. 1 was revised into the positive text with the word “unless” used to indicate it remains an exception. Former Exception No. 2 was also placed as positive text as follows: “Where space limitations prevent wiring from being routed a distance of 1.5 m (5 ft) or more from the pool, such wiring shall be permitted where installed in complete raceway systems of rigid metal conduit, intermediate metal conduit, or a nonmetallic raceway system. All metal conduit shall be corrosion resistant and suitable for the location. The minimum cover depth shall be as given in Table 680.10.” Again, the exception still remains and would have been much better as a separate paragraph or even made into a separate subsection with the first sentence inserted as (A) and the second sentence as (B).
The new text added to Table 680.10 in the 2011 NEC states: “Nonmetallic raceways listed for direct burial under minimum of 102 mm (4 in.) thick concrete exterior slab and extending not less than 162 mm (6 in.) beyond the underground installation shall be permitted at a 6 inches (150 mm) depth.” This new text permits the depth of the PVC conduit to be raised to 6 inches under a 4-inch concrete slab. The swimming pool lighting circuit must be GFCI-protected, further enhancing safety and preventing a shock hazard. A rewrite of this text for the 2017 NEC could clarify and ensure a contractor would not be required to bury the PVC conduit at 18 inches under a concrete deck.